1. Field of the Invention
The invention concerns optoelectronics and more particularly the production of passive or active photowritten components for use in optical telecommunications networks.
2. Description of the Related Art
Photowriting is widely used in the production of Bragg gratings on optical fibers or on silica-on-silicon planar guides.
A grating of this kind is produced by varying the luminous energy transmitted per unit surface area (referred to as the xe2x80x9cfluencexe2x80x9d) along the guide or the optical fiber.
In is often a question of creating interference fringes at a pitch xcex9 at the level of and normal to the guide (or the core of the fiber in the case of an optical fiber). This modulation of the illumination generates at the level of the guide spatial modulation of the refractive index along the direction of propagation of light and it is this spatial modulation that forms the Bragg grating.
As shown in FIG. 3, in its routine use for propagation, the photowritten guide (or fiber core) reflects a wavelength referred to as the Bragg wavelength (xcexB) whose value is given by the equation:
xcexB=2.N(eff).xcex9
where N(eff) is the effective refractive index of the guide and xcex9 is the pitch of the grating.
To obtain a Bragg grating, i.e. a modulated index grating, it is therefore necessary to expose the guide or the fiber to a UV beam modulated spatially along the guide.
Two interference systems are widely used to obtain spatial modulation of the UV beam, namely the phase mask and the Lloyd mirror. These systems are shown in figures 1 and 2.
Because the beam is narrow, the area of luminous interference formed by the exposure system is often shorter than the length required for the grating. This problem is solved by moving the combination of the guide and the interference system in front of the beam.
Photowriting a Bragg grating into silica doped with germanium leads to two phenomena that are relatively well known in the art, namely modulation of the index caused by the interference figure and an increase in the average index. These two effects are proportional, among other things, to the fluence of the UV exposure and the photosensitivity of the material.
Unfortunately, a constant amplitude of modulation like that shown in FIG. 4 leads to rejection and extinction spectra in the form of cardinal sinusoids (the Fourier transform of the modulation profile), with relatively large side lobes, which are undesirable, especially in Bragg gratings of wavelength division multiplex network components. FIG. 4b shows one such undesirable spectrum.
Generally speaking, a substantially Gaussian modulated refractive index profile and a constant average index are required, so that the reflection peak characteristic of the grating has the narrowest possible spectrum and no side lobes. Such modulation adaptation is referred to as xe2x80x9capodizationxe2x80x9d.
To eliminate the side lobes it is known in the art to adopt a modulation profile (or envelope) that is substantially Gaussian and to retain a constant average index (see FIG. 6).
In the case of a Gaussian profile average index, as shown in FIG. 5, a Fabry-Pxc3xa9rot cavity can be created between the edges of the grating, which then have a shorter Bragg wavelength than that reflected at the center (xcexB=2xcex9neff), as shown in FIG. 5b. This phenomenon is reflected in the occurrence of unwanted side lobes at the shorter wavelengths, which disappear if the average index becomes uniform again along the grating.
The appendix refers to a series of publications relating to apodization techniques known in the art.
The solution adopted for fibers is to photowrite the Gaussian modulated part of the index on a face of the fiber referred to as the front face, through the phase mask, and simultaneously to correct the average index continuously, with the aim of rendering it constant, on a face referred to as the rear face.
The Gaussian envelopes of the modulated index and continuous correction are obtained as the fiber and the mask are moved in front of the beam by spatial variation of the intensity of the laser beam. As shown in FIG. 7, the variation is obtained by placing two additional mask D1 and D2 fastened to the fiber in front of each beam, the additional masks having a transmission ratio that varies with position on each mask.
The combination D1, D2, the phase mask and the fiber move in front of the two laser beams during photowriting and the index modulation envelope is given by the two distribution densities over D1 and D2.
The above technique applies only to optical fibers, because it is impossible to photowrite through silicon for a silica-on-silicon guide. However, it would be possible to produce the apodized grating in two passes, one with a mask D1 and the phase mask and then the other with the mask D2 without the phase mask. A major problem would remain, namely the non-coincidence of the densities over D1 and D2 during the two photowriting passes.
A second apodizing solution consists of using the phase mask and vibrating the fiber or the phase mask using a piezo-electric system, with a vibration amplitude varying from the edge to the center of the photowritten grating such that the average exposure at the edges is quasi-continuous and the fluence of the fringes falls off towards the center and is eliminated at the center.
The second technique can be applied to a silica-on-silicon guide because everything happens on the front face, but it is more difficult to vibrate a silicon wafer and vibrating the wafer entails the risk of breaking the adhesive bonds between a fiber and a guide, as usually encountered in silica guide assemblies.
In both cases the index distribution along the axis of the fiber is preferably apodized with the envelope shown in FIG. 6.
The document FR 2 764 394 proposes to add a rotary mask in the form of a helix in front of a phase mask to interrupt the beam intermittently as it rotates.
The rotary mask is also adapted to move transversely to the beam and the average ratio of masking of the beam is higher or lower according to the distance of the beam from the center of the rotary mask. A modulation of the fluences applied to each guide portion is chosen by varying the transverse position of the rotary mask relative to the beam during the movement of the guide in front of the beam.
Although the above method gives satisfactory results, it is relatively complex to put into practice and the accuracy of the resulting apodization leaves room for improvement.
This improvement is the object of the invention, which proposes an apodization method that is easy to implement on a guide, in particular on a silica guide, and which is particularly precise.
The above object is achieved in accordance with the invention by an exposure method for producing a Bragg grating on a photosensitive guide or an optical fiber, in which method the guide (the fiber) is scanned by a light beam and means are provided for modulating the exposure time along the guide (the fiber) by varying the speed at which the beam moves along the guide (the fiber) so that it is located opposite each location of the guide (the fiber) for a time period that varies with the location, the method including the step of disposing in front of the guide (the fiber) a system adapted to create interference fringes on the guide (the fiber) and to scan the beam over the interference system at a speed that is modulated along the system, and furthermore the step of having the beam scan the guide (the fiber) at a modulated speed without the interference system on the path of the beam, the scanning with the interference system being effected with modulation of the exposure time increasing the exposure time in the central part of the guide (the fiber) and the scanning without the interference system being effected with modulation of the exposure time reducing the exposure time in the central part of the guide (the fiber).